PROJECT SUMMARY (See instructions); Subjects diagnosed with ARDS exhibit ~40% mortality, thus demanding fresh approaches to the management of this serious condition. This Project will investigate a totally new mechanism of both G+ and G- bacteria-induced endothelial barrier dysfunction in human endothelial cells and in ALI/ARDS. It follows the studies on the crucial role RhoA/Rac1 imbalance in endothelial barrier dysfunction and ALI/ARSD, proposed in Project 1. Here we focus on the activation of endothelial RhoA that produces endothelial hyperpermeability in culture and increased pulmonary capillary permeability, in vivo. We propose a novel mechanism of regulating RhoA activity that could be useful in the management of ALI and ARDS. Preliminary data from Project 1 and published studies suggest that pp60src is a key step in RhoA activation, which leads to the phosphorylation of the small heat shock protein 27 (hsp27), a major cause of F-actin stress fiber formation and endothelial barrier dysfunction. The kinase, ppGOsrc is a well-known heat shock protein 90 (hsp90) client protein and we recently published that hsp27 co-immunoprecipitates with hsp90. Preliminary data suggest that the hsp90 inhibitor, 17-/\AG reduces both LPS-induced ppSOsrc activation and hsp27 phosphorylation, as well as LPS-induced RhoA activation, in endothelial cells. Furthermore, we have recently demonstrated that hsp90 inhibition prevents and reverses LPS-induced endothelial barrier dysfunction, in culture, and reduces capillary hyper-permeability, inflammation, lung dysfunction and mortality in a mouse model of LPS-induced ALI. Still, the effects of hsp90-mediated regulation of ppBOsrc, RhoA and hsp27 activation on G+ and G- induced endothelial barrier dysfunction, especially human endothelial cell barrier function, and in the management of ALI, in vivo, remain unknown. This project will test the hypothesis that hsp90 is an important regulator of human endothelial cell hyper-permeability, in vitro, and of ALI, in vivo. We will further test the hypothesis that hsp90 exerts these actions, in part, by controlling the fate of two key proteins (ppSOsrc, hsp27) that are involved in RhoA activation and signal transduction. These studies represent an exciting new possibility in the management of ALI/ARDS.